FEATURE ARTICLE
The Evolutionary Ecology of Escherichia coli
Abundantly studied and much feared, E. coli has more genomic plasticity than once believed and may have followed various routes to become a pathogen
Valeria Souza, Amanda Castillo, Luis Eguiarte
E. coli in Its Environment
In spite of the abundance of bacteria, the study of their ecology is extraordinarily difficult and generally relies on indirect measurement. We believe the best way to understand bacterial ecology is through the use of genetic markers and the techniques of two fields: population genetics and molecular evolution.
Traditionally the colons of mammals and birds have been regarded as the natural habitat of E. coli. When E. coli causes illness, fecal contamination of food or water is commonly suspected; that is, health workers look for a way that E. coli from one mammalian colon could have gotten introduced into another's digestive system. It was long believed that the bacteria cannot reproduce on external media. However, recent results indicate that there are strains of E. coli that occupy niches other than the colon. Prominent among these are the pathogenic E. coli that can live in other parts of the digestive tract, in the blood, in the urogenital tract and in secondary environments. Strains found in drains and aquatic environments are in general more diverse than strains obtained directly from hosts.
A number of studies have found that aquatic and soil bacterial populations can increase their density over time, indicating that they grow and survive in these external environments. These studies suggest that E. coli infections can come from sources other than fecal contamination. But life for E. coli in a nutrient-poor environment such as water or mud is not the same as life in the rich environment of the mammalian gut. Bacteria in nutrient-poor environments divide at around 10 percent of the rate achieved in the laboratory.
Biologists know a number of things about the population ecology of E. coli that live in commensal relations with host animals. Generally there is one dominant strain of E. coli per host, but the appearance of new genotypes indicates that this dominance is temporary. Traditional evolution is at work here: The population can change through adaptive processes, in which a better strain displaces a less competitive one, or via the random processes known as genetic drift. The primary weapons of intraspecific competition are the colicins, which can destroy strains of the same species that do not display a plasmid coding for the same colicin. Colicins act by disrupting critical cellular functions such as the production of adenosine triphosphate, or ATP, the energy molecule central to cell metabolism.
E. coli is one of the first bacterial species to colonize mammals after birth; it is acquired from the birth canal and from the mother's feces. It has been calculated that the density of E. coli in the large intestine of mammals and birds is from 1 million to 10 million cells per gram of colon. This makes E. coli a minor component of the microbiota of this part of the intestine, which is primarily anaerobic, and has a total bacterial density calculated at some 100 billion cells per gram of colon. It is believed that in the intestine there is one cell division daily, whereas in the middle of a rich culture medium in the laboratory, E. coli K-12 can be seen to double six times a day or more.
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